Input/output peripheral equipment, including such computer items as printers, scanners and display devices require intermittent servicing by a host processor in order to ensure proper functioning. Services, for example, may include data delivery, data capture and/or control signals. Each peripheral will typically have a different servicing schedule that is not only dependent on the type of device but also on its programmed usage. The host processor is required to multiplex its servicing activity amongst these devices in accordance with their individual needs while running one or more background programs. Two methods for advising the host of a service need have been used: polled device and device interrupt methods. In the former method, each peripheral device is periodically checked to see if a flag has been set indicating a service request, while, in the latter method, the device service request is routed to an interrupt controller that can interrupt the host, forcing a branch from its current program to a special interrupt service routine. The interrupt method is advantageous because the host does not have to devote unnecessary clock cycles for polling. It is this latter method that the present invention addresses. The specific problem addressed by the current invention is the management of interrupts in a multi-processor system environment.
Multi-processor systems, often a set of networked computers having common peripheral devices, create a challenge in the design of interrupt control methods. For instance, in the case of a computer network servicing a number of users, it would be highly desirable to distribute the interrupt handling load in some optimum fashion. Processors that are processing high priority jobs should be relieved of this obligation when processors with lower priority jobs are available. Processors operating at the lowest priority should be uniformly burdened by the interrupt servicing requests. Also, special circumstances may require that a particular I/O device be serviced exclusively by a preselected (or focus) processor. Thus, the current invention addresses the problem of optimum dynamic and static interrupt servicing in multi-processor systems.
Prior art devices, exemplified by Intel's 82C59A and 82380 programmable interrupt controllers (PICs), are designed to accept a number of external interrupt request inputs. The essential structure of such controllers, shown in FIG. 1, consists of six major blocks:
______________________________________ IRR - Interrupt Request Register 11 stores all interrupt levels (IRQx) on lines 16 requesting service; ISR - Interrupt Service Register 12 stores all interrupt levels which are being serviced, status being updated upon receipt of an end-of-interrupt (EOI); IMR - Interrupt Mask Register 13 stores the bits indicating which IRQ lines 16 are to be masked or disabled by operating on IRR 11; VR - Vector Registers 19, a set of registers, one for each IRQ line 16, stores the preprogrammed interrupt vector number supplied to the host processor on data bus 17, containing all the necessary information for the host to service the request; PR - Priority Resolver 15, a logic block that determines the priority of the bits set in IRR 11, the highest priority is selected and strobed into the corresponding bit of ISR 12 during an interrupt acknowledge cycle (INTA) from the host processor; Control Logic - Control logic 20 coordinates the overall operations of the other internal blocks within the same PIC, activates the host input interrupt (INT) line 21 when one or more bits of IRR 11 are active, enables VR 19 to drive the interrupt vector onto data bus 17 during an INTA cycle, and inhibits all interrupts with priority equal or lower than that being currently serviced. ______________________________________
Several different methods have been used to assign priority to the various IRQ lines 16, including:
1) fully nested mode, PA1 2) automatic rotation--equal priority devices, made and PA1 3) specific rotation--specific priority mode. PA1 1) multiple I/O peripheral devices, each with its own set of interrupts; PA1 2) static as well as dynamic multi-processor interrupt management including the symmetrical distribution of interrupts over selected processors; PA1 3) level or edge triggered interrupt request pins, software selectable per pin; PA1 4) per pin programmable interrupt vector and steering information; PA1 5) programmable vector address field defined by each operating system; PA1 6) inter-processor interrupts allowing any processor to interrupt any other for dynamic reallocation of interrupt tasks; and PA1 7) support of system wide support functions related to non-maskable interrupts (NMI), processor reset, and system debugging. PA1 1) an I/O MPIC unit for acquiring interrupt request (IRQ) signals from its associated I/O peripheral devices, having a redirection table for processor selection and vector/priority information; PA1 2) local-MPIC units which may be separate auxiliary units connected to the associated processor or units that are partially or totally integrated into the associated processor, each managing interrupt requests for a specific system processor including pending, nesting and masking operations, as well as inter-processor interrupt generation; and PA1 3) a dedicated I/O PA1 -bus, distinct from any system or memory bus, for communications between the I/O and local MPIC units as well as between local-MPIC units.
The fully nested mode, supports a multi-level interrupt structure in which all of the IRQ input lines 16 are arranged from highest to lowest priority: typically IRQ0 is assigned the highest priority, while IRQ7 is the lowest.
Automatic rotation of priorities when the interrupting devices are of equal priority is accomplished by rotating (circular shifting) the assigned priorities so that the most recently served IRQ line is assigned the lowest priority. In this way, accessibility to interrupt service tends to be statistically levelled for each of the competing devices.
The specific rotation method gives the user versatility by allowing the user to select which IRQ line is to receive the lowest priority, all other IRQ lines are then assigned sequentially (circularly) higher priorities.
From the foregoing description it may be seen that PIC structures of the type described accommodate uni-processor systems with multiple peripheral devices but do not accommodate multi-processor systems with multiple shared peripheral devices to which the present invention is addressed.